Fix premature compaction: decouple ratio from tool tokens, raise thresholds (#4812)

* Fix premature compaction: decouple ratio from tool tokens, raise thresholds

- Compute compaction ratios against baseBudget instead of budgetThreshold
  so users with many tools aren't penalized with earlier compaction
- Reduce safety multiplier from 0.85 to 0.90 (10% covers observed
  tokenizer discrepancy without wasting 15% of context)
- Raise background compaction kick-off from 75% to 80%

Fixes microsoft/vscode#304475

* Address PR feedback: clamp messageBudget, clarify toolTokens guard comment

Author: bhavyaus

src/extension/intents/node/agentIntent.ts

@@ -395,7 +395,6 @@ export class AgentIntentInvocation extends EditCodeIntentInvocation implements I
 			throw new Error(`Setting github.copilot.${ConfigKey.Advanced.SummarizeAgentConversationHistoryThreshold.id} is too low`);
 		}
 
-		// Reserve extra space when tools are involved due to token counting issues
 		const baseBudget = Math.min(
 			this.configurationService.getConfig<number | undefined>(ConfigKey.Advanced.SummarizeAgentConversationHistoryThreshold) ?? this.endpoint.modelMaxPromptTokens,
 			this.endpoint.modelMaxPromptTokens
@@ -405,8 +404,14 @@ export class AgentIntentInvocation extends EditCodeIntentInvocation implements I
 		const summarizationEnabled = this.configurationService.getConfig(ConfigKey.SummarizeAgentConversationHistory) && this.prompt === AgentPrompt && !responsesCompactionContextManagementEnabled;
 		const backgroundCompactionEnabled = summarizationEnabled && this.configurationService.getExperimentBasedConfig(ConfigKey.BackgroundCompaction, this.expService);
 
-		const budgetThreshold = Math.floor((baseBudget - toolTokens) * 0.85);
-		const safeBudget = useTruncation ? Number.MAX_SAFE_INTEGER : budgetThreshold;
+		// When tools are present, apply a 10% safety margin on the message portion
+		// to account for tokenizer discrepancies between our tool-token counter and
+		// the model's actual tokenizer. Without this, an undercount could cause an
+		// API-level context_length_exceeded error instead of a graceful
+		// BudgetExceededError from prompt-tsx. When there are no tools the endpoint's
+		// own modelMaxPromptTokens is used unchanged.
+		const messageBudget = Math.max(1, Math.floor((baseBudget - toolTokens) * 0.9));
+		const safeBudget = useTruncation ? Number.MAX_SAFE_INTEGER : messageBudget;
 		const endpoint = toolTokens > 0 ? this.endpoint.cloneWithTokenOverride(safeBudget) : this.endpoint;
 
 		this.logService.debug(`AgentIntent: rendering with budget=${safeBudget} (baseBudget: ${baseBudget}, toolTokens: ${toolTokens}), summarizationEnabled=${summarizationEnabled}`);
@@ -436,12 +441,12 @@ export class AgentIntentInvocation extends EditCodeIntentInvocation implements I
 		//   ≥ 95% + InProgress             → block on the background compaction
 		//                                    completing, then apply before rendering.
 		//
-		//   ≥ 75% + Idle (post-render)     → kick off background compaction so
+		//   ≥ 80% + Idle (post-render)     → kick off background compaction so
 		//                                    it is ready for a future iteration.
 		//
 		const backgroundSummarizer = backgroundCompactionEnabled ? this._getOrCreateBackgroundSummarizer(promptContext.conversation?.sessionId) : undefined;
-		const contextRatio = backgroundSummarizer && budgetThreshold > 0
-			? this._lastRenderTokenCount / budgetThreshold
+		const contextRatio = backgroundSummarizer && baseBudget > 0
+			? (this._lastRenderTokenCount + toolTokens) / baseBudget
 			: 0;
 
 		// Track whether we applied a summary in this iteration so we don't
@@ -625,8 +630,8 @@ export class AgentIntentInvocation extends EditCodeIntentInvocation implements I
 
 		// 3. Post-render background compaction checks.
 		if (backgroundCompactionEnabled && backgroundSummarizer && !summaryAppliedThisIteration) {
-			const postRenderRatio = budgetThreshold > 0
-				? result.tokenCount / budgetThreshold
+			const postRenderRatio = baseBudget > 0
+				? (result.tokenCount + toolTokens) / baseBudget
 				: 0;
 
 			if (postRenderRatio >= 0.95 && backgroundSummarizer.state === BackgroundSummarizationState.InProgress) {
@@ -654,8 +659,8 @@ export class AgentIntentInvocation extends EditCodeIntentInvocation implements I
 					this.logService.debug(`[Agent] post-render background compaction finished but produced no usable result`);
 					this._sendBackgroundCompactionTelemetry('postRenderBlocked', 'noResult', postRenderRatio, promptContext);
 				}
-			} else if (postRenderRatio >= 0.75 && (backgroundSummarizer.state === BackgroundSummarizationState.Idle || backgroundSummarizer.state === BackgroundSummarizationState.Failed)) {
-				// At ≥ 75% with no running compaction (or a previous failure) — kick off background work.
+			} else if (postRenderRatio >= 0.80 && (backgroundSummarizer.state === BackgroundSummarizationState.Idle || backgroundSummarizer.state === BackgroundSummarizationState.Failed)) {
+				// At ≥ 80% with no running compaction (or a previous failure) — kick off background work.
 				this._startBackgroundSummarization(backgroundSummarizer, props, endpoint, token, postRenderRatio);
 			}
 		}